This function displays the FFT grid used on a plot of an lgcpPredict object.
First plot the object using for example plot(lg), where lg is an object
of class lgcpPredict, then for any of the plots produced, a call to
showGrid(lg,pch=="+",cex=0.5) will display the centroids of the FFT grid.
# S3 method for lgcpPredict
showGrid(x, ...)an object of class lgcpPredict
additional arguments passed to points
plots grid centroids on the current graphics device